1. A radioimmunoassay has been developed for measuring endothelin-like immunoreactivity in human plasma using an antibody raised against endothelin-1 which also cross-reacts with big endothelin-1 and endothelin-2 but not endothelin-3.

2. The sensitivity of the assay was 1 fmol/tube with inter- and intra-assay coefficients of variation of 13% and 9%, respectively. Cross-reactivity with endothelin-3 and non-endothelin peptides was less than 1%.

3. Endothelin-like immunoreactivity was present in the plasma of hypertensive patients (n = 25) at a concentration of 5.7 ± 0.5 pmol/l (mean ± sem), which was not significantly different from that of age-matched control subjects (5. ± 10.5 pmol/l). At these levels, endothelin-1 is unlikely to function as a circulating hormone.

4. Within the normotensive group, the concentration of endothelin-like immunoreactivity in plasma was positively correlated with mean arterial blood pressure, but in hypertensive patients it showed a significant negative correlation.

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